399/400 Troubleshooting

If you are stuck at 399/400, do not re-search the entire library at random. Find the section without a blue plaque, then inspect only that section.

Checklist

Walk both floors and identify the one section plaque that is not blue.
If you suspect books disappeared rather than being misfiled, go to the back landing between floors and use the little blue crystal above the podium. It summons missing books and explicitly says it will not count as magic.
Inside that section, count each same-title row against the expected volume count in its book page.
Look for same-title volumes split across two tiers.
Look for a title that belongs to a nearby theme but not this section.
Open that title's book page and use Where To Look First only as a broad hint for where loose copies may have started.
Pick up suspicious books and place them again; red glow confirms a bad placement.
If magic is allowed, use Shelf Guide on suspicious books and Insight on incomplete same-title groups.

Problem	Use this	Why it is safe
One or two books seem gone after checking furniture, upper shelves, and the relevant section.	Blue crystal above the podium on the back landing between floors.	Its in-game tooltip says it does not count as magic usage, and community no-magic runners confirm it solved missing-book cases.
A section plaque is not blue but all volumes are visible.	Manual row inspection.	Most 399/400 cases are still a wrong order, split title row, or nearby wrong-category title.
You are not on an Anti-Magic run.	Shelf Guide and Insight.	Major magic is the fastest way to locate unclear titles when achievements are not restricting it.